Virology Core C is the central point of the Program Project for specimen handling, storage and distribution from the domestic Clinical Core B (Figure 1). Scientifically, it is devoted to definition and dissection of HIV-1 levels, resistance states and the anatomic locations of HIV among participants enrolled and followed in this Program. This core is essential to and tightly integrated with the other cores and projects. After delivery from the clinic the samples are stored in the repository and can be retrieved and routed to the requestor quickly. Viral load results are sent to Admin Core A and Clinical Core B. Currently, the repository occupies five -80 freezers and four LN2 tanks. While existing space can handle additional plasma samples, another LN2 tank is required to store new cell samples that will be collected. Much of the emphasis of our work over the last 5 years has been to define sites and mechanisms of HIV-1 persistence during the course of antiretroviral therapy. We were first to show that viral latency in T cells is established early after infection^ and one of the first groups to demonstrate that continued low-grade virus replication is confined to specific cell types during the course of HAART^'. Understanding what factors influence persistence of HIV-1 replication is an ongoing emphasis of our laboratories'^. Many of the specific aims in this program are directed at virological and immunological factors that influence HIV-1 dynamics. As such, we have spent considerable time developing quantitative, reproducible, and sensitive assays that can measure HIV-1 at different stages of the virus cycle and in a variety of tissues and cell types.